It's an easy misunderstanding. If Donut makes it to at least the 4th floor, Mordecai will be given enough credit to be let out of his contract when the dungeon is over, regardless of how Carl and Donut fare from here. He is still Donut's manager as long as she is in the dungeon but now the pressure is off for him in regards to having to stay on for another season. He will be given more compensation the deeper they manage to go, but the 4th floor was the minimum to satisfy his contract this season.